This invention relates to a method for producing nitride monocrystals pursuant to Patent claim 1.
The production of high-efficiency structural elements such as semiconductor lasers and light-emitting diodes requires substrate crystals with maximized crystal perfection. Especially structural elements based on nitride semiconductors, for example such as Ga(In,Al)N, with which the ultraviolet to blue-green spectral region of the visible spectrum has very recently been opened up, can meet this requirement only conditionally, since the necessary semiconductor layer sequences are deposited on so-called foreign substrates (sapphire; SiC; Si; GaAs; etc.). However, perfect crystals require the use of xe2x80x9chomosubstrates,xe2x80x9d or substrates of the same or closely related material, to realize the low dislocation densities required for the structural elements.
Nitride crystals have very high bond energies. To produce corresponding substrate crystals, therefore, extremely high pressures and temperatures of about 1000xc2x0 C. and higher are necessary. The existing methods are based either on methods that enable the formation of crystals from the melt at high pressure, or on a sublimation method in which crystal growth occurs essentially from the gas phase. Quasi-crystals also lead to reduction of dislocation densities, but because of the lattice misfits physically present, they contain prestresses that can be eliminated only by the interfering dislocations. Such quasi-crystals are produced by deposition of thick nitride films on foreign substrates (like sapphire; SiC; Si; GaAs; etc.). Examples of methods used are organometallic gas phase epitaxy and hydride gas phase epitaxy.
The methods described here, on the one hand, are very costly and tedious, and on the other hand they do not provide the desired crystal perfection.
The underlying objective of this invention is thus to described a method for producing nitrides, especially (Ga, Al)Ni [sic], that on the one hand is as economical as possible, and on the other hand permits the most perfect possible monocrystalline structure.
The method pursuant to the invention for solving this problem is based on the thermal reaction and decomposition (pyrolysis) of an organic substance that contains the atomic constituents of the nitride monocrystal to be formed. This organic substance is contained in a solution or in a melt that is held at a first temperature. In the solution of the melt there is a substrate nucleus consisting of the nitride material to be grown or of a related type of nitride material. This substrate nucleus is supplied with thermal energy so that a second temperature is reached, at least on its surface in contact with the melt, that is higher than the first temperature. Because of this, nitride molecules are formed and deposited on the heated surface, and the nitride monocrystal thus grows.
In papers by R. Riedel et al. (Z. anorg. allg. Chem. 603 (1991), 119-127) and W. Rodger Nutt et al. (Inorg. Chem. 1985, 24, 159-164), it was shown, for example, that AlN and GaN xe2x80x9cmicrocrystalsxe2x80x9d can be produced from the thermal reaction of various organic substances (bis(dichloro-N-trimethylsilyl)cycloaminoalane; (trimethylsilyl)amino- and methyl(trimethylsilyl)aminogallium dichloride). The objective of the method described here is selective thermal decomposition on a crystal surface. For this purpose, the desired material or a related type of material is made available as a xe2x80x9ccrystal nucleus.xe2x80x9d The selective heating of just this surface leads to the decomposition of the organic compounds that are either in a melt or in a solution. Nitride molecules are formed that contain all of the atoms necessary for crystal construction. Heating the surface of the nucleus leads to the molecules formed also being able to be deposited only there, which leads to crystal growth. Such a method, moreover, is economical and requires only small expenditures for apparatus.
It is preferred for the solution or melt containing the organic substance to be in a container, and for the crystal nucleus to be positioned along a section of the container wall, with the thermal energy being fed to the substrate nucleus through this section of the container wall.
The thermal energy can be introduced in different ways. For example the substrate nucleus can be impacted by radiation emitted by a radiation source, particularly an infrared radiation source. However, the thermal energy can also be supplied inductively. Another possibility consists of introducing the thermal energy through a resistance heater. This can contain resistance wires that run in the section of the container wall and can be supplied with electrical current.